6 yrs ago my brother Brian, 16 yrs younger than me, had a motorcycle accident, at only 25 miles an hour, without a helmet. He tucked and rolled into the only object within a hundred feet in either direction which was a tree. After six years recovery, he is fairly ambulatory, but has the motivation and cognition of an 8-12 year old. We have no idea if he will ever reach independence and he's now 50 years old.
Brian is a wonderful person and I happily am his caretaker for what may be the rest of his life. But when I hear people say wearing a helmet doesn't have anything to do with anyone else, I'm pretty quick to point out the error in that thinking. It affects everyone around you. Today Brian will tell you, "I can't believe I wasn't wearing a helmet."